ANSI C78.377 is the publication from National Electrical Manufacturer’s Association (NEMA) that defines correlated color temperatures (CCTs) for LEDs. C78.377 has defined 10 CCTs (2200, 2500, 2700, 3000, 3500, 4000, 4500, 5000, 5700, and 6500 K). The recently released 2024 update adds definitions for CCTs of 2000 K and 1800 K for some outdoor and special indoor applications.
Earlier this month, CIE published CIE Position Statement on Color Quality Metrics, in which it recommends the lighting industry transition from the outdated and sometimes inaccurate General Color Rendering Index (CRI) to the General Color Fidelity Index (Rf ) defined in CIE 224:2017. The position statement notes that problems with CRI (which we’ve known about for years) include use of an outdated color space (CIE 1960 (u, v)), the small number of samples used to calculate CRI (only 8), and that CRI has proven to be especially problematic in evaluating narrow band emitters.
The good news is that CIE is finally recommending retiring CRI from use (which was last updated over 50 years ago in 1974) and adopting a modern, accurate metric for evaluating fidelity for all lighting applications. That’s a huge step forward for the lighting industry.
The less good news is that it stops there. The position statement acknowledges that fidelity is not the only aspect of color rendering, and that studies have shown preferences for light sources that slightly enhance saturation (and therefore reduce fidelity). However, it makes no mention of other metrics (such as ANSI/IES TM-30’s Gamut Index and Preference Design Intent) that address the issue. Since CIE 227’s Rf and TM-30’s Rf are identical, I see this as a belated endorsement of Rf as a fidelity metric and of TM-30 in general. My hope is that this spurs the industry to greater adoption of TM-30, especially for its evaluation of color preference, vividness, and fidelity described in Annex E.
One side note: Rf as defined in CIE 227 is Rf as defined in TM-30. In fact, TM-30 was published two years before CIE 227, which was a response to TM-30. In evaluating TM-30, CIE found that there were a few places where CIE and IES chose different methods of extrapolating certain information. Since CIE had formalized their procedures and IES had not, the two organizations worked together to harmonize their calculations into one calculation that is used in both systems. It’s frustrating that the CIE position statement reads as if CIE developed Rf out of whole cloth, rather than as a response to TM-30 and a mutual refinement of the Rf calculation.
At this point no one should be specifying fluorescent lamps except in extremely specific situations (for example, I recently specified linear fluorescents and linear LED retrofit lamps in a themed environment set in the 1970s). Why? Not only are LEDs more energy efficient, but more bans go into effect today prohibiting the sale of fluorescent lamps, including the following:
California: Compact Fluorescent (CFL) lamps with pin-bases and linear fluorescent lamps. Note that a screw base CFL ban is already in effect.
Colorado: CFL screw-base lamps, CFL pin-base lamps, and linear fluorescent lamps.
Hawaii: CFL screw-base lamps.
Minnesota: CFL screw-base lamps.
Oregon: CFL pin-base lamps and linear fluorescent lamps.
Note that a screw base CFL ban is already in effect.
Rhode Island: CFL pin-base lamps and linear fluorescent lamps. Note that a screw base CFL ban is already in effect.
For additional information LEDvance has a table of bans, current and future, for all states and Canada here.
If you look up the definition of Correlated Color Temperature (CCT) in IES LS-1 you’ll find, “The absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source.” It seems straightforward. The spectra of non-incandescent light sources don’t exactly match a blackbody radiator. They’ll plot off the blackbody locus in a chromaticity diagram. A CCT calculation identifies the color temperature closest to the light source in question – that’s the CCT. We all know that.
Not long ago someone on the IES Color Committee suggested that we develop and issue a standardized CCT calculation. My first thought was, “What? Are you saying that after decades of specifying CCT there’s no industry standard calculation method? What’s going on?” It turns out that since the late 1930s at least a dozen CCT calculation methods have been developed but none of them have been adopted by a standards setting body like the IES or CIE. The same is true for Duv, the direction and distance between the chromaticity coordinates of the light source and the nearest point on the blackbody locus. This means that the method used is up to the LED or equipment manufacturer, and can vary from one manufacturer to another.
Now, for lighting designers this isn’t a problem. Variations between the calculation methods generally aren’t large (although they can range from less than 1/1000th to several hundred K), and we are selecting LEDs described by their nominal CCT as outlined in NEMA C78.377, not their exact CCT. Those chromaticity quadrangles are huge, roughly 400 – 500 K wide and over 7 MacAdam ellipses, as shown below. For lighting designers the problem is the huge variation possible within a single CCT designation, but that’s for another day.
But, for LED manufacturers, testing equipment manufacturers, and researchers this can be a real problem. If two people attempt to precisely measure the same LED and arrive at two different CCTs how would they determine who’s right, or which calculation is “better”?
Now we have an answer. ANSI/IES TM-40 IES Method for Determining Correlated Color Temperature (CCT) and Distance from the Planckian Locus of Light Sources describes a CCT calculation method with an error of less than 0.1 K that calculates Duv based on the result of the CCT calculation. While there’s no way to compel anyone to use this method, it is an American National Standard developed under the ANSI process, making it the closest thing we have to an industry standard. Finally.
Here’s an interesting article about color perception, specifically blue/green perception, from The Guardian. There’s also a link to a fun web site where you can test your blue/green perception.
Source: Do you see blue or green? This viral test plays with color perception | Well actually | The Guardian
On Wednesday, September 25th I’ll be on a panel at the Retail Design Institute New York Chapter’s Lighting Summit 2024. The other panelists are Shoshanna Segal of Hantranft Lighting Design, Carlotta Dove of IA Interior Architects, and Kazumi Tanimura of Atelier Lumiere. The event is at the Illuminations showroom at 5 Penn Plaza. Hope to see you there!
I’ll be giving a presentation on TM-30 (naturally) at the American Lighting Association Conference in Palm Beach, FL on September 23rd. If you’re at the conference and/or a fan of TM-30 stop by and say hi.
We’re now up to 14 states and Canada that have enacted one or more restrictions or bans on mercury containing lamps. Some of the bans are already active, while others go into effect as early as January 1, 2025. LEDVANCE is maintaining a list on their website along with other helpful resources.
The big fluorescent news last week was that Minnesota has become the ninth state to pass a phase out of fluorescent lamps (the others are California, Colorado, Hawaii, Maine, Oregon, Rhode Island, Vermont, and Washington). Minnesota’s timeline is aggressive – screw-based compact fluorescent lamps will be banned from sale six months from now, starting on January 1, 2025. Other fluorescent and mercury vapor lamps will be banned from sale beginning January 1, 2026.
The Minnesota House of Representatives has an information sheet about the new law here. What’s interesting is that the first topic is about reducing the hazard of exposure to mercury, not increasing energy efficiency. It’s not wrong, just curious. The information sheet then goes on to address reduced CO2 emissions, electricity savings, and reduced power plant mercury emissions.
Illinois also has fluorescent banning legislation working through the legislature, so by the end of the year we may see 10, 11 or more states that are phasing out fluorescents.
It’s been a while since I’ve posted anything because I’ve been working on a new book. More on that as we get closer to the publication date. Meanwhile, I’ve been asked (somewhat at the last minute) to speak on Monday June 3rd at an upcoming AIA New York event called Healing Light: The Biological and Social Effects of Lighting. The presentation will be an introduction to color temperature and color rendering and is primarily targeted at architects and interior designers who are new to lighting. Registration is free to AIA members and students, and only $10 for the general public.